There are several known methods for the aerosolized delivery of drugs. In general, the methods include: (1) placing an aqueous formulation within a nebulizer device which by various mechanical means causes the drug formulation to be aerosolized in a continuous stream which is inhaled by the patient; (2) dry powder inhalers which create a fine powder of the drug and aerosolize the powder in a dust form which is inhaled; (3) metered dose inhalers which dissolve or disperse the drug in a low boiling point propellant; and (4) more current devices such as that disclosed within U.S. Pat. No. 5,660,166 issued Aug. 26, 1997 which force aqueous formulations through a nozzle to create an aerosol which is inhaled by the patient.
In accordance with each of the known methods for aerosolizing a drug it is important to produce an aerosol which has particles within a desired size range, e.g. 0.5 to 12.0 microns and more preferably 1.0 to 3.5 microns. In addition to producing small particles it is preferable to produce particles which are relatively consistent in size, i.e. produce an aerosol wherein a large percentage of the particles fall within the desired size range. In addition, it is desirable to produce an aerosol which has the property that the key measures of aerosol quality, such as particle size and dose emitted are not effected by ambient conditions such as temperature and or relative humidity. With any of the known methods for aerosol delivery of drugs there are difficulties with respect to making the particles sufficiently small. Along with these difficulties there are difficulties with respect to creating particles which are relatively consistent in size. These difficulties are particularly acute when attempting to provide for systemic delivery of an aerosolized drug. Efficient systemic delivery requires that the aerosol be delivered deeply into the lung so that the drug can efficiently reach the air/blood exchange membranes in the lung and migrate into the circulatory system.
Aerosol delivery to the lungs has been used for delivery of medication for local therapy (Graeser and Rowe, Journal of Allergy 6:415 1935). The large surface area, thin epithelial layer, and highly vascularized nature of the peripheral lung (Taylor, Adv. Drug Deliv. Rev. 5:37 1990) also make it an attractive site for non-invasive systemic delivery. Unlike other avenues of non-invasive delivery such as trans-dermal, nasal, or buccal, the lung is designed as a portal of entry to the systemic circulation. However, targeting the peripheral lung requires careful control of the aerosol particle size and velocity distributions, in order to by pass the exquisitely evolved particle filtering and clearing functions of the bronchial airways.
Many authors have reported results of experiments or mathematical models showing that micron sized particles are required for delivery to the lungs (c.f. Stahlhofen, Gebhart and Heyder, Am. Ind. Hyg. Assoc. J. 41:385 1980, or Ferron, Kreyling and Haider, J. Aerosol Sci. 19:611 1987). One example is the model of the Task Group on Lung Dynamics (Morrow et. al. Health Physics 12: 173 1966). As FIG. 1 shows, under the assumptions of this model, particles of diameter less than .about.3.5 .mu.m are required to avoid the oropharynx and bronchial airways. FIG. 1 might suggest that the maximum efficiency of deposition of drugs delivered to the pulmonary region of the lung is limited to .about.60%. However, as can be seen in FIG. 2, efficiencies approaching 100% can be achieved by allowing the particles to settle gravitationally during a ten second breath hold (Byron, J. Pharm. Sci. 75:433 1986).
It has been demonstrated that ambient conditions can strongly effect the amount of aerosol particles less than 3.5 .mu.m emitted from aerosol generation device. One example is the work of Phipps and Gonda (Chest 97:1327-1332, 1990) showing that the amount of aerosol less than 3.5 .mu.m delivered by a aerosol drug delivery device changed from 33% to 73% when the relative humidity changed from 100% to 70%. Similar work with a dry powder (Hickey et al J. Pharm. Sci. 79, 1009-1011) demonstrated a change in the amount of aerosol less than 3.5 .mu.m from 9% to 42% when the ambient relative humidity changed from 97% to 20%. These data are tabulated in Table 1.
TABLE 1 ______________________________________ Effect of RH on Particle Size Distribution Aerosol T, .degree. C. R. H., % % &lt; 3.5 .mu.m ______________________________________ Isotonic Saline.sup.1, Hudson Up-Draft 23-24.degree. 100% 33% Isotonic Saline.sup.1, Hudson Up-Draft 23-24.degree. 65-75% 73% Fluorescein Powder.sup.2 37 .+-. 0.1.degree. 97 .+-. 1% 9% Fluorescein Powder.sup.2 37 .+-. 0.1.degree. 20 .+-. 5% 42% ______________________________________ 1: Phipps and Gonda, 1990 2: Hickey et al 1990
Many pharmaceutical compounds of a wide range of molecular weights are potential candidates for systemic delivery via the lung. Small molecules analgesics such as morphine or fentanyl could be delivered to pain patients, e.g. cancer or post-operative patients. Morphine has demonstrated bioavailability when delivered via the lung (S. J. Farr, J. A. Schuster, P. M. Lloyd, L. J. Lloyd, J. K. Okikawa, and R. M. Rubsamen. In R. N. Dalby, P. R. Byron, and S. J. Farr (eds.), Respiratory Drug Delivery V Interpharm Press, Inc., Buffalo Grove, 1996, 175-185).
Potent peptide hormones are available for a variety of therapeutic indications. Leuprolide, for example, is a GnRH super-agonist useful in the treatment of endometriosis and prostate cancer. Leuprolide also has potential applications in the field of breast cancer management and the treatment of precocious puberty. Calcitonin enhances metabolism and may be a useful therapeutic agent for the management of osteoporosis, a common complication of aging.
To treat conditions or diseases of the endocrine system, pharmaceutical formulations containing potent peptide hormones are typically administered by injection. Because the stomach presents a highly acidic environment, oral preparations of peptides are unstable and readily hydrolyzed in the gastric environment. Currently, there are no oral preparations of therapeutic peptide agents commercially available.
Both calcitonin and leuprolide can be administered nasally. (See Rizzato et al., Curr. Ther. Res. 45:761-766, 1989.) Both drugs achieve blood levels when introduced into the nose from an aerosol spray device. However, experiments by Adjei et al. have shown that the bioavailability of leuprolide when administered intranasally is relatively low. However, an increase in the bioavailability of leuprolide can be obtained by administering the drug into the lung. Intrapulmonary administration of leuprolide has been shown to be an effective means of non-invasive administration of this drug (Adjei and Garren, Pharmaceutical Research, Vol. 7, No. 6, 1990).
Intrapulmonary administration of drugs has the advantage of utilizing the large surface area available for drug absorption presented by lung tissue. This large surface area means that a relatively small amount of drug comes into contact with each square centimeter of lung parenchyma. This fact reduces the potential for tissue irritation by the drug and drug formulation. Local irritation has been seen with nasal delivery of insulin and has been a problem for commercialization of nasal preparations of that drug. It is a problem with peptide hormones that they are very potent with effects that are not immediately manifested. For example, therapy with leuprolide for prostate cancer does not typically produce any acute clinical effects. Similarly, prophylaxis against osteoporosis with calcitonin will not produce any acute symptoms discernible to the patient. Therefore, administration of each dose of these drugs must be reliable and reproducible.